Optical switching by controllable frustration of total internal reflection

ABSTRACT

An optical switch for controllably switching an interface between a reflective state in which incident light undergoes total internal reflection and a non-reflective state in which total internal reflection is prevented. In one such switch an elastomeric dielectric has a stiffened surface portion. A separator positioned between the interface and the stiffened surface portion maintains a gap there-between. A voltage source applies a variable voltage potential between electrodes on the interface and stiffened surface portion respectively. The applied voltage potential moves the stiffened surface portion into optical contact with the interface, producing the non-reflective state. In the absence of a voltage potential the separator moves the stiffened surface portion away from optical contact with the interface, producing the reflective state. In another such switch a cell contains a fluid. One side of the cell forms the light incident interface. A membrane is suspended in the fluid. One pair of electrodes is applied to opposite sides of the membrane. Another electrode pair is applied to the cell&#39;s interface side and to the cell&#39;s opposite side. A variable voltage potential is applied between selected ones of the electrodes. Application of the voltage potential between selected ones of the membrane and cell electrodes moves the membrane into optical contact with the interface, producing the non-reflective state. Application of the voltage potential between other selected ones of the membrane and cell electrodes moves the membrane away from optical contact with the interface, producing the reflective state.

REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. application Ser. No. 08/923,431filed Sep. 4, 1997, now Pat. No. 5,999,307

TECHNICAL FIELD

This application pertains to a method and apparatus for frustrating thephenomenon of total internal refection in a continuously variable,easily controllable manner.

BACKGROUND

It is well known that light travels at different speeds in differentmaterials. The change of speed results in refraction. The relativerefractive index between two materials is given by the speed of anincident light ray divided by the speed of the refracted ray. If therelative refractive index is less than one, as in the case when lightpasses from glass block to air, then a light ray will be refractedtowards the surface. Angles of incidence and reflection are normallymeasured from a direction normal to the interface. At a particular angleof incidence “i”, the refraction angle “r” becomes 90° as the light runsalong the block's surface. The critical angle “i” can be calculated, assin i=relative refractive index. If “i” is made even larger, then all ofthe light is reflected back inside the glass block and none escapes fromthe block. This is called total internal reflection. Because refractiononly occurs when light changes speed, it is perhaps not surprising thatthe incident radiation emerges slightly before being totally internallyreflected, and hence a slight penetration (roughly one micron) of theinterface, called “evanescent wave penetration” occurs. By interferingwith (i.e. scattering and/or absorbing) the evanescent wave one mayprevent (i.e. “frustrate”) the total internal reflection phenomenon.

In a number of applications, it is desirable to controllably frustratethe phenomenon of total internal reflection. For example, if totalinternal reflection is occurring at an interface “I” as shown in FIG.1A, the extent of such reflection can be reduced by placing a dielectricmaterial “D” close to interface I, such that dielectric D interacts withthe evanescent wave penetrating beyond interface I, as shown in FIGS.1B, 1C, and 1D, in which the extent of frustration of total internalreflection is gradually increased, culminating in complete frustration(FIG. 1D).

It is desirable that dielectric D be an elastomeric material.Inevitably, at least some foreign particles “P” (FIG. 2A) are trappedbetween dielectric D and interface I; and/or, the opposing surfaces ofdielectric D and interface I have at least some dimensionalimperfections “X” (FIG. 2B) which prevent attainment of a high degree ofsurface flatness over substantial opposing areas of both surfaces. Suchforeign particles, or such surface imperfections, or both, can preventattainment of “optical contact” between dielectric D and interface I.Optical contact brings dielectric D substantially closer than one micronto interface I, thereby scattering and/or absorbing the evanescent waveadjacent interface I, thus preventing the capability of interface I tototally internally reflect incident light rays. If dielectric D isformed of an elastomeric material, the aforementioned adverse effects ofsuch foreign particles and/or surface imperfections are localized,thereby substantially eliminating their impact on attainment of thedesired optical contact. More particularly, as seen in FIGS. 2C and 2D,the elastomeric nature of dielectric D allows dielectric D to closelyconform itself around foreign particle P and around surface imperfectionX, such that optical contact is attained between dielectric D andinterface I except at points very close to foreign particle P and aroundsurface imperfection X. Since such points typically comprise only a verysmall fraction of the opposing surface areas of dielectric D andinterface I, sufficiently substantial optical contact is attained tofacilitate frustration of total internal reflection as described above.

Elastomeric materials vary considerably in surface tack, but virtuallyall are too tacky to be practical for this application withoutmodification. This is because most elastomeric materials aresufficiently soft and have enough surface energy that the material candeform into intimate “atomic contact” with the atomic scale structurepresent at any surface. The resulting Van der Waals bonding issufficient to make it difficult to remove the material from the surface.

It is desirable to provide a means for controlling frustration of totalinternal reflection by varying an interfacial pressure applied betweendielectric D and interface I; and, in general, it is desirable tominimize the applied pressure. The aforementioned Van der Waals bondingcan require negative pressures of order 10⁴ Pascals for release, whichis desirably reduced. Further, it is desirable to separate dielectric Dand interface I by an amount exceeding the evanescent wave zone when theapplied pressure is removed. The present invention addresses thesedesires.

SUMMARY OF INVENTION

The invention provides an optical switch for controllably switching aninterface between a reflective state in which light incident upon theinterface undergoes total internal reflection and a non-reflective statein which total internal reflection is prevented at the interface. In oneembodiment, the switch incorporates a preferably elastomeric dielectrichaving a stiffened surface portion. A separator is positioned betweenthe interface and the stiffened surface portion to maintain a gapthere-between. Electrodes are applied to the interface and stiffenedsurface portion respectively. A voltage source controllably applies avariable voltage potential between the electrodes. Application of avoltage potential between the electrodes moves the stiffened surfaceportion into optical contact with the interface, producing thenon-reflective state at the interface. In the absence of a voltagepotential between the electrodes the separator moves the stiffenedsurface portion away from optical contact with the interface, producingthe reflective state at the interface.

The separator may be a plurality of stand-offs provided at spacedintervals between the interface and the stiffened surface portion tomaintain the gap at about 1 micron in the absence of a voltage potentialbetween the electrodes. Advantageously, the stand-offs are an integralpart of the interface.

The dielectric's surface may be stiffened by applying to it a thin filmmaterial having a Young's Modulus value substantially less than thedielectric's Young's Modulus value. Alternatively, and to better enablethe dielectric's surface to flex in the vicinity of the standoffs, thedielectric's surface may be stiffened by applying a thin layer of hardparticles thereto.

In another embodiment, the optical switch incorporates a cell containinga fluid. One side of the cell forms the interface upon which light isincident. A membrane is suspended in the fluid. One pair of electrodesis applied to opposite sides of the membrane; and, another electrodepair is applied to the side of the cell forming the interface and to thecell's opposite side. A variable voltage potential is applied betweenselected ones of the electrodes. Application of the voltage potentialbetween selected ones of the membrane and cell electrodes moves themembrane into optical contact with the interface, producing thenon-reflective state at the interface. Application of the voltagepotential between other selected ones of the membrane and cellelectrodes moves the membrane away from optical contact with theinterface, producing the reflective state at the interface.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A, 1B, 1C and 1D show various stages in frustration of the totalinternal reflection phenomenon at interface “I” as dielectric “D” isgradually moved toward interface I.

FIGS. 2A and 2B respectively depict a foreign particle “P” and a surfaceimperfection “X” preventing attainment of optical contact betweeninterface I and dielectric D.

FIGS. 2C and 2D respectively depict attainment of substantial opticalcontact between interface I and dielectric D notwithstanding foreignparticle P or surface imperfection X if dielectric D is an elastomericmaterial.

FIGS. 3A and 3B depict a stiff-surfaced non-adhesive elastomericdielectric positioned adjacent an interface in accordance with oneoptical switch embodiment of the invention. FIG. 3A depicts the “off”state in which stand-offs maintain a gap between the interface anddielectric in the absence of an applied pressure, allowing totalinternal reflection to occur. FIG. 3B depicts the “on” state in whichelectrodes applied to the interface and dielectric are actuated to applya controllably variable pressure, closing the gap sufficiently tofrustrate total internal reflection.

FIG. 3C is similar to FIGS. 3A and 3B, but depicts an alternatetechnique for stiffening the surface of the elastomeric dielectric byapplying a layer of hard, sub optical size particles thereto. Thistechnique resists undesirable adhesion between the dielectric andinterface without impairing the dielectric's ability to flex in regionsproximate to the stand-offs. The left hand portion of FIG. 3C depictsthe “off” state in which stand the offs maintain a gap between theinterface and dielectric in the absence of an applied pressure, allowingtotal internal reflection to occur. The right hand portion of FIG. 3Cdepicts the “on” state in which an applied pressure has closed the gapsufficiently to frustrate total internal reflection.

FIG. 4 is a graph on which percentage surface reflectivity is plotted asa function of pressure applied between the dielectric and interface ofdepicted in FIGS. 3A and 3B.

FIGS. 5A and 5B depict an alternate optical switch embodiment of theinvention, with FIG. 5A depicting the “off” state in which totalinternal reflection occurs, and FIG. 5B depicting the “on” state inwhich total internal reflection is frustrated.

DESCRIPTION

FIG. 3A depicts an elastomeric dielectric 10 positioned adjacentinterface 12. As depicted, interface 12 is one face of a prism 13. Prism13 may be but one of a very large number of 90° prisms in a sheet ofprismatic film such as 2370 3M optical lighting film. Alternatively,prism 13 may be a 55° prism formed of a high refractive index material.However, prisms are not essential to the invention; interface 12 couldalternatively be one face of a slab waveguide or other structure capableof totally internally reflecting light rays at interface 12. Light raysincident (14A) upon interface 12 are totally internally reflected (14B)because air gap 16 between the opposing surfaces of dielectric 10 andinterface 12 is large enough to prevent optical contact between theopposing surfaces (i.e. gap 16 is substantially greater than onemicron). As hereinafter explained, dielectric 10 is fabricated such thatthe Young's Modulus E of dielectric 10 varies as a function of distancefrom the surface of dielectric 10 adjacent interface 12, such that aportion 18 of dielectric 10 near the surface is substantially stifferthan in the remaining portions of dielectric 10.

The stiffened surface portion 18 of dielectric 10 prevents attainment ofthe aforementioned Van der Waals bonding between dielectric 10 andinterface 12, since such bonding occurs only if dielectric 10 issufficiently deformable. Roughly speaking, the Young's Modulus of amaterial (a measure of the material's stiffness) must be less than theVan der Waals bond energy per unit area divided by a characteristicdimension associated with the material's surface roughness, in order forsubstantial atomic contact to occur. If dielectric 10 and interface 12are sufficiently smooth to exhibit total internal reflection (i.e. ifthe surface roughness dimensions characterizing dielectric 10 andinterface 12 are substantially less than one micron) and if dielectric10 and interface 12 exhibit typical surface energies, then undesirableadhesion occurs between dielectric 10 and interface 12 if the Young'sModulus of dielectric 10 is less than about 10⁶ Pascals, which is thecase for elastomeric materials. Hence, by increasing the Young's Modulusof elastomeric dielectric 10 at the surface of dielectric 10 one maystiffen that surface sufficiently to prevent undesirable adhesionbetween dielectric 10 and interface 12.

The aforementioned surface stiffening should be such that the surface ofdielectric 10 can assist in achieving a predictable, reproducible degreeof frustration of total internal reflection which varies as a functionof the pressure applied between dielectric 10 and interface 12.Preferably, under low positive interfacial pressure, the degree offrustration of total internal reflection is low, and air gap 16 retainsa well defined average width of slightly over one micron. This isimportant, particularly if the interfacial pressure is to be created byelectrostatic attraction, as such narrow width air gaps can supportlarge electric fields due to the “Paschen effect”, and these largefields can be produced with comparatively low voltages, due to the smallgap width.

One method of stiffening the surface of elastomeric dielectric 10 is toprepare a uniform, smooth-surfaced elastomeric material, and then treatthat material in a manner which stiffens a thin surface portion of thematerial. For example, an elastomeric material can be initially hardened(“stabilized”) by exposure to ultraviolet light, or by application ofchemical cross linking agents. A thin film formed of a material having aYoung's Modulus much higher than that of the elastomeric material canthen be deposited on the elastomeric material's stabilized surface. Asone example, an indium tin oxide film can be deposited on a stabilizedelastomeric dielectric surface. The deposited film not only stiffensthin surface portion 18 of elastomeric dielectric 10 as aforesaid, butalso functions as a transparent surface electrode 20 for applying avariable electrostatic pressure between dielectric 10 and interface 12as hereinafter explained. An opposing electrode 22 can be applied tointerface 12 in well known fashion. An insulating film 24 such as zincoxide or vacuum deposited parylene can also be applied to the stabilizedsurface of dielectric 10 to act as an insulator between theaforementioned electrodes and/or to stiffen dielectric 10 to prevent Vander Waals bonding and thus prevent undesirable adhesion betweendielectric 10 and interface 12.

As seen in FIGS. 3A and 3B, a separator such as a plurality of rigidstand-offs 26 are provided at spaced intervals between dielectric 10 andinterface 12; and, more particularly, between electrodes 20, 22.Stand-offs 26 are each about 1 micron high and serve to maintain gap 16between dielectric 10 and interface 12 at about 1 micron if voltagesource “V” is not actuated to apply a voltage between electrodes 20, 22which is sufficient to cause the attractive electrostatic pressurebetween electrodes 20, 22 to move electrode 20 and dielectric 10 intogap 16 and into optical contact with interface 12 as seen in FIG. 3B.Thus, stand-offs 26 serve to maintain gap 16 between dielectric 10 andinterface 12 in the absence of an applied pressure, allowing totalinternal reflection to occur as illustrated by reflected ray 14B in FIG.3A. However, when a voltage is applied as aforesaid to move electrode 20and the stiffened bulk surface portion 18 of dielectric 10 into gap 16,total internal reflection is frustrated as illustrated by non-reflectedray 28 in FIG. 3B. The invention thus provides an optical switch, withFIGS. 3A and 3B respectively depicting the “off” and “on” states.

The embodiment of the invention described above with reference to FIGS.3A and 3B may “over stiffen” surface portion 18 of dielectric 10,leaving dielectric 10 with insufficient surface flexibility for adequatedeformation of dielectric 10 in regions proximate to stand-offs 26.Consequently, when a voltage is applied as aforesaid to move dielectric10 into gap 16, dielectric 10 may not extend sufficiently into gap 16 tofrustrate total internal reflection in regions proximate to stand-offs26, causing visually perceptible light or dark spots to appear in thevicinity of stand-offs 26. An alternate stiffening method can be used toovercome this potential deficiency.

Specifically, as shown in FIG. 3C, a layer of hard, suboptical size(i.e. less than 1 micron in diameter) particles 30 can be applied to thesurface of dielectric 10 to produce stiffened surface portion 18.Particles 30 can be made from any one of a number of substances,including ceramics or hard polymers, provided particles 30 aresufficiently small that the bulk of the particulate layer is able toflex sufficiently to extend inside the evanescent wave zone in regionsproximate to stand-offs 26 when pressure is applied to move dielectric10 into gap 16 as aforesaid. The FIG. 3C embodiment thus providesanother optical switch, with the left and right hand portions of FIG. 3Crespectively depicting the “off” and “on” states.

The height of stand-offs 26 and/or the spacing between adjacent pairs ofstand-offs 26 can be altered during fabrication of optical switchesembodying the invention to vary the force which stand-offs 26 exert inthe absence of an applied pressure. Preferably, only the spacing betweenadjacent pairs of stand-offs 26 is altered, since this does not affectthe attractive electrostatic pressure exerted between electrodes 20, 22by actuation of voltage source “V”. Standoffs 26 can be directlyfabricated on the surface of interface 12, for example byphoto-developing a film applied to the surface of interface 12, with thefilm's thickness determining the height of stand-offs 26.

A desirable property of the optical switches depicted in FIGS. 3A, 3Band 3C is that a gradual increase in the attractive electrostaticpressure exerted between electrodes 20, 22 by actuation of voltagesource “V” produces a corresponding gradual increase in the extent offrustration of total internal reflection at interface 12. Moreparticularly, by suitably varying the voltage applied between electrodes20, 22 one may vary the displacement between stiffened surface 18 ondielectric 10 and interface 12 within a continuously variable range ofoptical contact values, thereby attaining any desired degree offrustration of the capability of interface 12 to totally internallyreflect incident light rays. FIG. 4 graphically illustrates theresultant range of percentage reflectivity as a function of theattractive electrostatic pressure exerted between electrodes 20, 22. InFIG. 4, “P1” denotes the minimum pressure at which substantially allincident light is totally internally reflected, and “P2” denotes themaximum pressure at which total internal reflection is substantiallyfrustrated (i.e. substantially no incident light is totally internallyreflected).

In the embodiments of 3A, 3B and 3C, the Young's Modulus is increased inthe surface portion 18 of dielectric 10. More particularly, in surfaceportion 18 E>a/d, where E is the Young's Modulus within surface portion18, a is the bond energy per unit area due to the Van der Waals forcebetween interface 12 and dielectric 10, and d is a dimensioncharacteristic of surface roughness of interface 12. A comparable resultcan be obtained by reducing a, as will now be explained in relation toFIGS. 5A and 5B.

FIG. 5A depicts a cell 40 filled with fluid 42 having a low (less thanabout 1.3) index of refraction, such as 3M Flourinert™. A planarelastomeric membrane 44 bearing a first pair of opposed upper and lower(as viewed in FIGS. 5A and 5B) surface electrodes 46, 48 is suspendedwithin fluid 42. A second pair of insulated electrodes 50, 52 areprovided on the opposed internal upper and lower surfaces of cell 40.The thickness “T” of membrane 44 and its electrodes 46, 48 is a fewmicrons less than the width “W” of cell 40 (i.e. the perpendiculardisplacement between insulated electrodes 50, 52) so that reasonableelectrostatic pressures can be produced between adjacent electrodes tomove membrane 44, as hereinafter explained. Both the length and thedepth of membrane 44 and its electrodes 46, 48 are greater than thewidth “W” of cell 40, so that membrane 44 remains oriented as shown,with electrodes 46, 48 generally parallel to insulated electrodes 50,52.

Total internal reflection at interface 54 is controlled by means ofvoltage source “V”. Specifically, if voltage source “V” is actuated toapply an attractive electric field between electrodes 46, 50 whileelectrodes 48, 52 are at maintained at equal potential, then membrane 44is repelled away from and does not contact interface 54 as seen in Fig.5A, thus allowing total internal reflection to occur at interface 54 asillustrated by reflected ray 56. If voltage source “V” is actuated toapply an attractive electric field between electrodes 48, 52 whileelectrodes 46, 50 are maintained at equal potential, then membrane 44moves upwardly (as viewed in FIG. 5B) through fluid 42 into opticalcontact with interface 54, thus frustrating total internal reflection atinterface 54 as illustrated by non-reflected ray 58 in FIG. 5B. Cell 40thus constitutes an optical switch, with FIGS. 5A and 5B respectivelydepicting the “off” and “on” states. Fluid 42 reduces the relativesurface energy a, facilitating optical contact at interface 54 withoutadhesion. Use of a low refractive index fluid 42 in combination with ahigh refractive index optical medium 60 (e.g. a high index polymer orother transparent material having an index of refraction greater thanabout 1.65) reduces the critical angle “i” at which total internalreflection occurs, thus increasing the range of angles at which incidentlight can be totally internally reflected. This is an important factor,since it directly affects the acceptable range of viewing angles of adisplay incorporating a plurality of optical switches (i.e. cells 40).

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. For example, although dielectric 10 is prefer ably asilicone elastomer, it need not necessarily be an “elastomer”; it issufficient for the bulk dielectric material to be a reasonably flexiblesubstance, such as Teflon™. Further, persons skilled in the art willappreciate that the “on” and “off” optical switch states are arbitrary.Thus, the state depicted in any of FIG. 3A, the left side of FIG. 3C, or5A could be designated as the “on” state, with the opposite statedepicted in FIG. 3B, the right side of FIG. 3C, or 5B respectively beingdesignated as the “off” state. Similarly, different combinations ofattractive or repulsive electric fields can be applied between one orthe other of electrodes 46, 48 and one or the other of electrodes 50, 52to move membrane 44 into or out of optical contact with interface 54.Accordingly, the scope of the invention is to be construed in accordancewith the substance defined by the following claims.

What is claimed is:
 1. Apparatus for controllably switching an interfacebetween a reflective state in which light incident upon said interfaceundergoes total internal reflection and a non-reflective state in whichtotal internal reflection is prevented at said interface, said apparatuscomprising: (a) a dielectric having a stiffened surface portion; (b) aseparator positioned between said interface and said stiffened surfaceportion to maintain a gap there-between; (c) a first electrode on saidinterface; (d) a second electrode on said stiffened surface portion;and, (e) a voltage source for controllably applying a variable voltagepotential between said electrodes; wherein: (i) application of saidvoltage potential between said electrodes moves said stiffened surfaceportion into optical contact with said interface, producing saidnon-reflective state at said interface; (ii) in the absence of saidvoltage potential between said electrodes said separator moves saidstiffened surface portion away from optical contact with said interface,producing said reflective state at said interface.
 2. Apparatus asdefined in claim 1, wherein E>a/d, where E is said stiffened portion'sYoung's Modulus, a is the bond energy per unit area due to Van der Waalsbonding between said interface and said member, and d is a dimensioncharacteristic of roughness of said interface.
 3. Apparatus as definedin claim 2, wherein said stiffened portion's Young's Modulus is greaterthan about 10⁶ Pascals.
 4. Apparatus as defined in claim 1, wherein saiddielectric is an elastomer.
 5. Apparatus as defined in claim 4, whereinsaid separator further comprises a plurality of stand-offs provided atspaced intervals between said interface and said stiffened surfaceportion.
 6. Apparatus as defined in claim 5, wherein said gap is about 1micron.
 7. Apparatus as defined in claim 5, wherein said stand-offs eachhave a height of about 1 micron.
 8. Apparatus as defined in claim 5,wherein said stand-offs are an integral part of said interface. 9.Apparatus as defined in claim 5, wherein said stiffened surface portionfurther comprises a thin layer of hard particles.
 10. Apparatus asdefined in claim 4, wherein said particles have an average diameter lessthan 1 micron.
 11. Apparatus as defined in claim 4, wherein saidstiffened surface portion further comprises a thin film material havinga first Young's Modulus value, said elastomer having a second Young'sModulus value substantially less than said first Young's Modulus value.12. Apparatus as defined in claim 11, wherein said thin film material isindium tin oxide.
 13. Apparatus as defined in claim 11, wherein saidthin film material further comprises said second electrode. 14.Apparatus as defined in claim 4, further comprising an insulatorpositioned between said electrodes.
 15. Apparatus as defined in claim14, wherein said insulator further comprises a thin film material. 16.Apparatus as defined in claim 15, wherein said insulator furthercomprises said stiffened surface portion.
 17. Apparatus as defined inclaim 1, wherein said dielectric is a silicone elastomer.
 18. Apparatusas defined in claim 1, wherein said voltage source is controllable tomove said stiffened surface portion into optical contact with saidinterface within a continuously variable range of optical contactvalues.
 19. A method of controllably switching an interface between areflective state in which light incident upon said interface undergoestotal internal reflection and a non-reflective state in which totalinternal reflection is prevented at said interface, said methodcomprising: (a) producing said non-reflective state at said interface bycontrollably applying a variable voltage potential between saidinterface and an adjacent stiffened surface portion of a dielectric tomove said stiffened surface portion into optical contact with saidinterface; and, (b) producing said reflective state at said interface byremoving said voltage potential to move said stiffened surface portionaway from optical contact with said interface.
 20. A method as definedin claim 19, wherein said dielectric is an elastomer.
 21. A method asdefined in claim 20, wherein said dielectric is a silicone elastomer.22. A method as defined in claim 20, further comprising forming a firstelectrode on said interface and forming a second electrode on saidstiffened surface portion for application of said voltage potentialbetween said electrodes.
 23. A method as defined in claim 22, furthercomprising maintaining a gap between said interface and said stiffenedsurface portion in the absence of said applied voltage potential.
 24. Amethod as defined in claim 23, wherein said gap is about 1 micron.
 25. Amethod as defined in claim 23, further comprising maintaining said gapby positioning a separator between said interface and said stiffenedsurface portion.
 26. A method as defined in claim 23, further comprisingmaintaining said gap by positioning a plurality of stand-offs betweensaid interface and said stiffened surface portion.
 27. A method asdefined in claim 26, wherein said stand-offs each have a height of about1 micron.
 28. A method as defined in claim 26, further comprisingforming said stand-offs integrally with said interface.
 29. A method asdefined in claim 26, further comprising depositing a thin layer of hardparticles on said dielectric to form said stiffened surface portion. 30.A method as defined in claim 29, wherein said particles have an averagediameter less than 1 micron.
 31. A method as defined in claim 22,further comprising forming said stiffened surface portion integrallywith said first electrode from a thin film material having a firstYoung's Modulus value, said elastomer having a second Young's Modulusvalue substantially less than said first Young's Modulus value.
 32. Amethod as defined in claim 31, wherein said thin film material is indiumtin oxide.
 33. A method as defined in claim 22, further comprisingforming an insulator between said electrodes.
 34. A method as defined inclaim 33, further comprising forming said insulator from a thin filmmaterial.
 35. A method as defined in claim 33, further comprisingforming said stiffened surface portion integrally with said insulator.36. A method as defined in claim 20, further comprising forming saidstiffened surface portion from a thin film material having a firstYoung's Modulus value, said elastomer having a second Young's Modulusvalue substantially less than said first Young's Modulus value.
 37. Amethod as defined in claim 36, wherein said thin film material is indiumtin oxide.
 38. A method as defined in claim 19, further comprisingvarying said voltage potential to move said stiffened surface portioninto optical contact with said interface within a continuously variablerange of optical contact values.